NCERT Solutions Class 10 Science Chapter 2 Acids, Bases and Salts

Plan the order of tests. Red litmus gives a clear yes-or-no answer for only one of the three liquids, so use the result of the first test to set up the second. Red litmus changes colour only when it meets a base.

Testing all three tubes with red litmus picks out the basic solution at once, because that is the only tube where the red paper turns blue. The acid and the water both leave red litmus unchanged, so after round one those two still look the same.

The clever part is that the strip which turned blue in the base is now, for all purposes, blue litmus paper, and blue litmus is exactly what finds an acid. Dip this freshly blued strip into the two unknown tubes. The one that turns it red is the acid, and the one that leaves it blue is the neutral distilled water. With a single supply of red litmus and a sensible order of testing, all three liquids are identified.

Answer: The basic tube turns red litmus blue; reuse that blue strip to find the acid (turns it red) and the water (no change).

Link the sour taste to a chemical reaction. The idea to pull out is that sour means acidic, and acids react with metals. Curd, lemon, tamarind and vinegar all owe their sour taste to acids: curd to lactic acid, lemon to citric acid, tamarind to tartaric acid, vinegar to acetic acid.

Copper and brass are reactive enough to be attacked by these mild acids over time. When the food acid sits in a copper or brass vessel, it slowly reacts with the metal surface and produces copper compounds. These copper salts are poisonous if eaten, and they also give the food an unpleasant taste and colour.

This is why sour foods are stored in vessels that do not react with acids, such as stainless steel, glass or glazed earthenware. The same reasoning explains why we should not squeeze lemon onto food kept in a brass plate for a long time. The chemistry is exactly the metal-with-acid reaction from this chapter, just happening slowly in the kitchen.

Answer: Acids in curd and sour foods react with copper and brass to give toxic copper salts that spoil the food, so non-reactive vessels are used instead.

Tie the answer to a balanced equation. A complete answer has three parts: the name of the gas, a balanced example reaction, and the test. Many students lose marks by skipping the balanced equation, so always include one.

The gas given off when a metal reacts with a dilute acid is hydrogen. A clean example is zinc dropped into dilute sulphuric acid, which fizzes and releases hydrogen while forming zinc sulphate. Writing this as Zn + H₂SO₄ → ZnSO₄ + H₂ shows the salt and the gas clearly. You could equally use magnesium with dilute hydrochloric acid, Mg + 2HCl → MgCl₂ + H₂; the gas is hydrogen in both cases.

To test the gas, collect it in a test tube and bring a burning matchstick or candle close to the mouth of the tube. Hydrogen catches fire and burns with a sharp pop sound. That pop is the simple, reliable confirmation that the gas is hydrogen, and it is the test the board expects you to describe.

Answer: Hydrogen is liberated; e.g. Zn + H₂SO₄ → ZnSO₄ + H₂. A burning matchstick near the gas makes it burn with a pop sound, confirming hydrogen.

Work backwards from the clues. This is a small detective problem. Read the three clues and let each narrow down compound A. Clue one: effervescence with dilute HCl means a gas is produced. Clue two: the gas extinguishes a flame, the signature of carbon dioxide. Clue three: one product is calcium chloride, so calcium must be in compound A.

Putting these together, a compound that releases carbon dioxide with an acid is a carbonate, and since the salt formed is calcium chloride, the compound is calcium carbonate, CaCO₃. Limestone, chalk and marble are all forms of CaCO₃, so this is a very common reaction.

Now write the equation and balance it carefully. Because one calcium chloride unit carries two chlorine atoms, you must use two molecules of HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. A quick atom count on both sides confirms the balance.

Answer: A is calcium carbonate; CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Acidity is about free H⁺ ions, not just hydrogen atoms. The whole answer rests on one idea: a compound is an acid only when it produces hydrogen ions in water. The presence of hydrogen atoms in the formula is not enough.

Hydrochloric acid and nitric acid are made of molecules that break apart in water and release H⁺ ions, which immediately join water molecules to form hydronium ions, H₃O⁺. These free hydrogen ions make the solution taste sour, turn blue litmus red and react with metals, so in water these substances are clearly acidic.

Alcohol and glucose tell a different story. Their molecules carry hydrogen atoms, but when they dissolve they stay as whole molecules and do not let go of any hydrogen as ions. With no free H⁺(aq) in the solution, there is nothing to produce acidic behaviour. This is exactly why a glucose drink is not sour, while even dilute hydrochloric acid is sharply acidic. The lesson is to test for the release of H⁺ ions, not just to look for an H in the formula.

Answer: Acids release H⁺(aq) ions in water; alcohol and glucose keep their hydrogen inside the molecule and release no H⁺, so they are not acidic.

Conduction needs moving charges. The cleanest way to think about this is to ask what carries the current. In a metal wire, electrons move. In a solution there are no free electrons, so the only way to carry charge is through ions that can drift through the liquid.

An acid dissolved in water ionises and fills the solution with free hydrogen ions and free negative ions. For hydrochloric acid these are H⁺(aq) and Cl⁻(aq). Because these ions are charged and free to move, they respond to the battery: positive ions drift one way and negative ions the other. This movement of charge is exactly an electric current, which is why the bulb glows.

By contrast, glucose and alcohol dissolve as neutral molecules and produce no ions, so their solutions have nothing to carry the charge and the bulb stays dark. The single deciding factor is whether the dissolved substance releases free ions. Acids do, so their aqueous solutions are good conductors.

Answer: Because the acid ionises in water to give free H⁺ and negative ions, and these moving ions carry the current.

No water, no ions, no acidity. The heart of this answer is that acidic behaviour comes from free hydrogen ions, and those appear only when water is around to separate them from the rest of the molecule.

Hydrogen chloride as a dry gas is made of whole HCl molecules. With no moisture present, these molecules stay intact and do not release any H⁺ ions. Litmus, however, responds to H⁺ ions. Dry litmus paper has no water in it either, so when dry HCl gas meets dry litmus there is simply no water anywhere to set the hydrogen ions free.

Since no H⁺ ions are produced, the paper detects no acid and keeps its original colour. The situation changes completely if the litmus is moist: the small amount of water lets HCl ionise on the paper, free H⁺ ions appear, and blue litmus promptly turns red. This shows that the acidic property of HCl depends on water being present, not on the gas alone.

Answer: Without water dry HCl gas cannot give H⁺ ions, and with no H⁺ ions there is no acidity, so dry litmus paper stays unchanged.

It is a heat-management problem. The reason for the rule comes straight from the fact that diluting a strong acid is highly exothermic, so the real question is how to stop the released heat from becoming dangerous.

Think about what each order does. If you pour water onto concentrated acid, a little water lands on a large amount of acid and the heat is released in that small patch of water. That water can flash to steam, and the sudden boiling throws hot acid droplets out of the container, which can burn skin and eyes and even crack the glassware.

Now reverse it. If you add the acid slowly to a large volume of water while stirring, each bit of acid is surrounded by plenty of water that soaks up the heat. The temperature climbs only gently because so much water shares the heat, and the mixing stays under control. This is why every laboratory rule and the warning labels on acid bottles tell you to add acid to water, never water to acid.

Answer: Because dilution is highly exothermic, acid is added slowly to a large amount of water so the heat is absorbed safely; the reverse order causes violent splashing of hot acid.

Same ions, more water. The way to reason about dilution is to track two things separately: how many hydronium ions there are, and how much liquid they sit in. Concentration is the first divided by the second.

When you add water to an acid, you are not adding any new acid, so the actual number of H₃O⁺ ions stays the same as before. What changes is the total volume of the solution, which goes up because you have poured in extra water.

Concentration is the number of ions in each unit of volume. With the count of H₃O⁺ ions unchanged but the volume larger, the number of ions packed into each millilitre goes down. So the hydronium ion concentration decreases on dilution. This is also why a diluted acid is milder and its pH moves closer to 7: fewer hydronium ions per unit volume means weaker acidic behaviour.

Answer: The hydronium ion concentration decreases on dilution, since the same number of H₃O⁺ ions is spread through a greater volume.

More base means more hydroxide ions. The simple principle is that the strength of a basic solution is set by how many hydroxide ions it holds in each unit of volume, and adding more base directly adds more of those ions.

Sodium hydroxide is a strong base, so when it dissolves it splits fully into sodium ions and hydroxide ions. The hydroxide ions make the solution basic and push the pH above 7. Start with a sodium hydroxide solution that already has a certain number of OH⁻ ions.

Now dissolve extra sodium hydroxide in the same solution. Each new unit of NaOH releases its own hydroxide ion, so the total number of OH⁻ ions climbs while the volume barely changes. The result is a higher concentration of hydroxide ions per unit volume, which means a more strongly basic solution and a higher pH.

Answer: The hydroxide ion concentration increases, because the extra base releases more OH⁻ ions into the solution.

Read the pH scale in two directions. Two small facts unlock this: pH 7 is the neutral mark, and pH falls as hydrogen ions rise. Holding both in mind makes the comparison quick.

Place A and B on the scale. Solution A sits at pH 6, just below neutral, and solution B sits at pH 8, just above neutral. Anything below 7 is acidic and anything above 7 is basic, so A is the acidic solution and B is the basic one.

For the hydrogen ion concentration, use the inverse relationship: a lower pH means more hydrogen ions. Since A has the lower pH, it must contain more hydrogen ions than B. So solution A is both the more acidic solution and the one with the greater hydrogen ion concentration, while B, being above 7, is basic and poorer in hydrogen ions. Keeping the rule lower pH, more H⁺ clear prevents the common mix-up of thinking a bigger pH number means more acid.

Answer: Solution A (pH 6) has more hydrogen ions and is acidic; solution B (pH 8) is basic.

Tie nature directly to the H⁺ count. The cleanest answer is to state plainly that the hydrogen ion concentration is what controls whether a solution is acidic, neutral or basic.

When a solution is rich in H⁺(aq) ions, it behaves as an acid: it tastes sour, turns blue litmus red, and sits below 7 on the pH scale. The more hydrogen ions per unit volume, the stronger the acidity and the lower the pH.

As the hydrogen ion concentration falls, the solution becomes less acidic. When the hydrogen ions are balanced exactly by hydroxide ions, the solution is neutral at pH 7, like pure water. If hydroxide ions outnumber hydrogen ions, the solution turns basic and the pH rises above 7. So a single quantity, the H⁺(aq) concentration, decides the whole character of the solution: high means acidic, balanced means neutral, low means basic.

Answer: A higher H⁺(aq) concentration makes the solution more acidic (lower pH); a lower H⁺ concentration makes it basic, and equal H⁺ and OH⁻ make it neutral.

It is a question of which ion wins. The key realisation is that acidity and basicity are not about the total absence of one ion, but about the balance between hydrogen ions and hydroxide ions.

In every aqueous solution, water itself supplies a small number of both H⁺(aq) and OH⁻(aq) ions, so a basic solution does indeed contain some hydrogen ions. The answer to the first part is therefore yes.

What makes the solution basic is the comparison between the two kinds of ions. In a basic solution the hydroxide ions are present in much larger numbers than the hydrogen ions. Since the OH⁻ ions are in clear excess, the solution feels soapy, turns red litmus blue and has a pH above 7. The few hydrogen ions that remain are simply overwhelmed by the larger crowd of hydroxide ions, so they do not give the solution acidic character.

Answer: Yes, basic solutions have a small amount of H⁺(aq) ions, but they are basic because OH⁻(aq) ions are present in much greater number.

Match the treatment to the problem. Each of the three substances named is basic, so the right question is: when does soil need a base added to it? The answer is when the soil has turned acidic.

Soil can become acidic for several reasons, such as overuse of certain fertilisers or natural processes, and acidic soil holds back the growth of most crops. To fix this, a farmer needs something that will react with and remove the extra acid, which is exactly what a base does in a neutralisation reaction.

Calcium oxide, calcium hydroxide and calcium carbonate are all bases that the farmer can spread on the field. They react with the acid in the soil and neutralise it, nudging the pH back up towards neutral. Once the soil is no longer too acidic, plants can take up nutrients properly and grow well. So the single condition is when the soil is too acidic; these treatments would not be added to soil that is already neutral or basic.

Answer: The farmer uses these basic substances when the soil is too acidic, so that they neutralise the excess acid and restore a pH suitable for plant growth.

Connect formula to common name. This is a recall question, and the best preparation is to keep a short list linking formulas to familiar names. The compound Ca(ClO)₂ appears in the chapter as the product made by treating dry slaked lime with chlorine.

That product is what we call bleaching powder. Although its exact composition is a little complex in practice, the chapter represents it by the formula Ca(ClO)₂, and its common name is simply bleaching powder. It is used for bleaching cloth and paper, as a disinfectant to make drinking water germ-free, and as an oxidising agent in industry.

Remembering this link also helps with related questions, such as how bleaching powder is prepared from chlorine and slaked lime, or why it smells of chlorine. Building these formula-to-name pairs for the common salts and their derivatives is one of the most reliable ways to secure easy marks in this chapter.

Answer: Ca(ClO)₂ is commonly known as bleaching powder.

Trace the preparation backwards. The question gives you the product, bleaching powder, and asks for the starting material that reacts with chlorine. So recall how bleaching powder is made in the chapter.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime. Slaked lime is the common name for calcium hydroxide, Ca(OH)₂. So the substance that yields bleaching powder on treatment with chlorine is slaked lime.

Writing the reaction makes this clear: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O, where CaOCl₂ (also written Ca(ClO)₂) is bleaching powder. Knowing both the name and the formula of slaked lime, and being able to write this single preparation equation, covers the whole family of bleaching-powder questions the board may ask, including its uses in disinfecting water and bleaching textiles.

Answer: Slaked lime, calcium hydroxide Ca(OH)₂, gives bleaching powder when treated with chlorine.

Pick the right sodium salt. The chapter lists several useful chemicals made from common salt, so the task is to choose the one whose job is softening hard water. That chemical is washing soda.

Washing soda is sodium carbonate with ten molecules of water of crystallisation, written Na₂CO₃·10H₂O. It is obtained by recrystallising the sodium carbonate that you get on heating baking soda. Among its listed uses, removing the permanent hardness of water is the one this question tests.

Hard water owes its hardness to dissolved calcium and magnesium compounds. When washing soda is added, it reacts with these compounds and takes the calcium and magnesium out of solution, leaving the water soft. This is why washing soda is the standard sodium compound named for softening hard water, and why it is also used in laundries and homes for cleaning. Stating the common name with its formula gives a full-mark answer.

Answer: Washing soda, sodium carbonate Na₂CO₃·10H₂O, is the sodium compound used for softening hard water.

Recognise it as thermal decomposition. The clue word is heated, so think of what heat does to baking soda. Sodium hydrogencarbonate is a salt that breaks apart when warmed, which is a thermal decomposition reaction.

When heated, one product is sodium carbonate, and the other two are water and carbon dioxide. The carbon dioxide gas bubbles off, which is exactly why baking soda is used in cooking: the escaping gas makes cakes and pakoras rise and become soft and spongy.

The balanced equation needs two molecules of sodium hydrogencarbonate on the left to match one sodium carbonate on the right: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, with the reaction happening on heating. Writing the equation balanced and noting that heat drives it shows you understand both the chemistry and its everyday use.

Answer: Heating sodium hydrogencarbonate gives sodium carbonate, water and carbon dioxide: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ on heating.

Setting is a chemistry of water of crystallisation. Plaster of Paris and gypsum are the same calcium sulphate, just with different numbers of water molecules attached. Plaster of Paris is the hemihydrate with half a water per unit, and gypsum is the dihydrate with two.

When you mix Plaster of Paris with water, the powder absorbs water molecules and converts back into gypsum. Because gypsum carries two waters and the plaster carries only half a water, the difference of one and a half water molecules has to be supplied by the added water, giving CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.

This conversion is what makes the wet paste set into a solid block, and it explains the storage rule: because Plaster of Paris reacts with water so readily, it has to be kept in a moisture-proof container, otherwise it would slowly set from the moisture in the air. Writing the equation with the correct water amounts is the key to a full-mark answer.

Answer: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O; Plaster of Paris takes up water to form gypsum and sets hard.

Translate the indicator clue into a pH range. For a multiple-choice question like this, convert the observation into a band on the pH scale first, then test the options against that band.

The single observation given is that the solution turns red litmus blue. Red litmus changes to blue only in the presence of a base, so straight away we know the solution is basic. Basic solutions all lie above pH 7 on the scale, which can run as high as 14.

Now scan the four options. Choices 1, 4 and 5 are all below 7, which would mean acidic solutions that turn blue litmus red, not the other way round, so they are ruled out. Only option (d), pH 10, lies above 7 and fits a basic solution. The general skill here, turning an indicator result into a pH range and then matching, makes these MCQs fast and safe.

Answer: Option (d) 10, because turning red litmus blue means the solution is basic with pH above 7.

Identify the gas, then the reactant. Two test results point the way: a gas is produced, and that gas turns lime water milky. The lime-water test is the classic confirmation of carbon dioxide, so the gas is CO₂.

Carbon dioxide is released when a carbonate meets an acid, and egg-shells are made mostly of calcium carbonate. So the unknown solution must be supplying the acid that attacks the carbonate of the shells. That immediately tells us to look for an acid among the four options.

Sodium chloride, lithium chloride and potassium chloride are all neutral salts and would not react with egg-shells to give a gas. Hydrochloric acid is an acid and reacts readily: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The carbon dioxide it produces then turns lime water milky. The reasoning chain, gas to CO₂ to carbonate-plus-acid to which option is an acid, is the reliable route.

Answer: Option (b) HCl, the only acid, which reacts with the carbonate of egg-shells to give CO₂ that turns lime water milky.

Treat it as a ratio problem. Because the same NaOH and the same HCl are used throughout, the volumes that exactly neutralise each other keep a fixed proportion. That single fact does all the work.

The starting information is that 10 mL of the sodium hydroxide solution is just neutralised by 8 mL of the hydrochloric acid solution. This sets the ratio of base to acid at 10 to 8 for these particular solutions.

Now the amount of base is increased to 20 mL, which is exactly twice the original 10 mL. Since the strengths have not changed, twice as much base needs twice as much acid. So the acid required becomes twice 8 mL, that is 16 mL, written as 8 × (20/10) = 16 mL. This matches option (d). Spotting that the base volume simply doubled lets you answer in seconds without any formal molarity calculation, which is the intended quick method for this kind of board MCQ.

Answer: Option (d) 16 mL, since doubling the NaOH volume doubles the HCl needed to neutralise it.

Match the medicine to the cause. The smart approach is to first work out what is wrong during indigestion, then choose the medicine that fixes that exact problem. Indigestion comes from the stomach producing too much hydrochloric acid.

If the trouble is excess acid, the cure must be something that neutralises acid, which means a base. The chapter calls such medicines antacids, literally against acid, and they are mild bases like magnesium hydroxide, milk of magnesia. They react with the surplus stomach acid and bring relief from the pain and burning.

The other three choices target different problems and do not neutralise acid: an antibiotic is for bacterial infections, an analgesic is a painkiller, and an antiseptic is applied to wounds to stop germs. None addresses excess stomach acid. So the medicine for indigestion is the antacid, option (c), and the reasoning excess acid needs a base makes the choice obvious.

Answer: Option (c) Antacid, a mild base that neutralises the excess stomach acid causing indigestion.

Follow one pattern, then balance. All four parts are the same kind of reaction: a dilute acid attacks a metal to give a salt and hydrogen gas. Write each word equation in that fixed shape first, naming the salt from the metal and the acid, and then balance the symbol equation.

For zinc and dilute sulphuric acid, the salt is zinc sulphate, giving Zn + H₂SO₄ → ZnSO₄ + H₂, already balanced. For magnesium with dilute hydrochloric acid, the salt is magnesium chloride and two HCl are needed for the two chlorines: Mg + 2HCl → MgCl₂ + H₂.

Aluminium needs the closest attention because it has a valency of three. Two aluminium atoms combine with three sulphate groups, so three sulphuric acid molecules are required: 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂. Finally iron with dilute hydrochloric acid forms iron(II) chloride: Fe + 2HCl → FeCl₂ + H₂. In every case the gas is hydrogen, confirmed by the pop test.

Answer: (a) Zn + H₂SO₄ → ZnSO₄ + H₂; (b) Mg + 2HCl → MgCl₂ + H₂; (c) 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂; (d) Fe + 2HCl → FeCl₂ + H₂.

Design the test around H⁺ ions. The aim is to prove that glucose and alcohol are not acids even though they contain hydrogen. Since acidity comes from free H⁺ ions, and free ions make a solution conduct electricity, an electrical conductivity test is the perfect activity.

Set up a simple circuit: two nails as electrodes dipped in the test liquid, connected through a bulb and a switch to a battery. The bulb is the detector; it lights up only when the liquid carries a current, which happens only when free ions are present. First test dilute hydrochloric or sulphuric acid, and the bulb glows, confirming that acids release H⁺ ions into the water.

Next, with the same circuit, test glucose solution and then alcohol solution. In both cases the bulb stays dark, showing that these liquids do not produce ions and therefore do not release H⁺ ions. Since the test for acidity fails for glucose and alcohol, they cannot be classed as acids despite having hydrogen in their molecules. The neat acid-versus-non-acid contrast in one simple experiment is what makes this the standard activity for the question.

Answer: A conductivity test (bulb glows for acids but not for glucose or alcohol) proves glucose and alcohol release no H⁺ ions, so they are not acids.

Compare what is dissolved in each. The deciding factor for whether water conducts is whether it holds free ions, so compare distilled water and rain water on that single point.

Distilled water has been purified by boiling and condensing, which removes dissolved salts, acids and gases. What is left is almost pure water molecules, with so few ions that no measurable current can pass. That is why distilled water behaves almost like an insulator and does not light the bulb in a conductivity test.

Rain water is a different case. As raindrops fall, they dissolve carbon dioxide and other acidic gases from the atmosphere, forming weak acids, and they also pick up traces of dissolved salts and dust. These dissolved substances release free ions into the rain water. With free ions available to move between the electrodes, rain water carries a current and conducts electricity. So the contrast comes entirely from the presence or absence of dissolved ions, not from the water itself.

Answer: Distilled water has no free ions and does not conduct; rain water dissolves acidic gases and salts that give it free ions, so it conducts.

Water is the key to ionisation. The whole answer turns on the fact that acidity is produced by free hydrogen ions, and HCl can give those ions only when water is present to pull the molecule apart.

In the dry state, hydrogen chloride exists as intact HCl molecules. There is no water to separate the hydrogen from the chlorine, so the molecules hold together and no H⁺ ions are set free. Acidic behaviour, such as turning litmus red or tasting sour, depends entirely on the presence of these hydrogen ions.

Because dry HCl gas produces no hydrogen ions, it cannot act as an acid; it shows no acidic behaviour at all. The moment water is introduced, the picture changes: the HCl reacts with water to form hydronium and chloride ions, HCl + H₂O → H₃O⁺ + Cl⁻, and the now-free hydrogen ions make the solution strongly acidic. This is why the same substance is inert as a dry gas but a strong acid in solution.

Answer: Dry HCl gas does not ionise without water, so it forms no H⁺ ions and shows no acidic behaviour.

Sort the values on the pH scale. The fastest way is to lay the five pH values out in order and read off each label from its position relative to 7. The values are A 4, B 1, C 11, D 7 and E 9.

Neutral is pH 7, so D is neutral at once. Anything below 7 is acidic and anything above 7 is basic. The most extreme low value, B at pH 1, is strongly acidic, while A at pH 4 is only mildly below 7 and so is weakly acidic. On the basic side, the highest value, C at pH 11, is strongly alkaline, and E at pH 9, just above 7, is weakly alkaline. That settles all five labels.

For the hydrogen-ion concentration order, recall the inverse rule: lower pH means more hydrogen ions. To list the concentration in increasing order, start from the highest pH and move to the lowest, giving pH 11, 9, 7, 4, 1, that is solutions C, E, D, A and B. Keeping the lower pH, more H⁺ rule in mind is the one step students most often reverse, so state it explicitly.

Answer: Neutral D, strongly alkaline C, strongly acidic B, weakly acidic A, weakly alkaline E; hydrogen-ion concentration increases as C (11) < E (9) < D (7) < A (4) < B (1).

Compare the supply of H⁺ ions. Because the metal, its length, and the amount and concentration of acid are all the same in the two test tubes, the only thing that can differ is how many hydrogen ions each acid provides. That is exactly the strong-versus-weak difference.

Hydrochloric acid is a strong acid, which means it ionises almost completely in water and floods the solution with hydrogen ions. Acetic acid is a weak acid, so even at the same concentration only a small fraction of its molecules ionise, and it releases far fewer hydrogen ions.

The fizzing is caused by hydrogen gas bubbling off as the acid attacks the magnesium, and the rate of that reaction rises with the number of available hydrogen ions. Since test tube A with HCl has many more hydrogen ions than test tube B with acetic acid, the magnesium in A reacts faster and the fizzing is more vigorous there. This experiment is a clean demonstration that the strength of an acid, set by how many H⁺ ions it gives, controls how fast it reacts, quite apart from its concentration.

Answer: Test tube A (HCl) fizzes more vigorously, because the strong acid HCl supplies more H⁺ ions than the weak acetic acid, so it reacts faster with magnesium.

Link the souring to acid formation. The change in pH follows directly from what happens chemically when milk sets into curd: bacteria make lactic acid. Adding acid to a liquid lowers its pH, so the pH must go down.

Fresh milk starts at pH 6, which already sits slightly on the acidic side of neutral. As it ferments into curd, microorganisms break down the milk sugar and release lactic acid into it. This new acid increases the number of hydrogen ions present in the milk.

Since the hydrogen ion concentration goes up, the pH comes down to a value below 6, and the curd is more acidic than the milk it came from. This is also why curd tastes distinctly more sour than fresh milk, because sourness is the taste of acidity. So the answer is that the pH decreases as milk turns into curd, and the reason is the production of lactic acid during the souring process.

Answer: The pH decreases below 6 as milk turns into curd, because lactic acid is produced, raising the hydrogen ion concentration.

One base, two consequences. Both parts follow from a single fact: baking soda is a mild base, and curd formation needs an acidic medium. Holding these two ideas together explains the milkman's trick.

For part (a), fresh milk is at pH 6, slightly acidic, and acidic milk tends to spoil and sour quickly. By stirring in a pinch of baking soda, the milkman adds a base that neutralises some of the acid and lifts the pH past 7 to slightly alkaline. In this less acidic state the milk resists souring and stays fresh for longer, which is why he deliberately makes the shift from 6 to slightly alkaline.

For part (b), curd sets only when bacteria have produced enough lactic acid to make the milk sufficiently acidic. When the milk has been made alkaline, the bacteria must first generate enough acid just to cancel the added base before the milk can even reach the acidity needed to curdle. This extra step takes time, so the treated milk takes much longer to set as curd. The same property that keeps the milk fresh, its raised pH, is what slows the curdling.

Answer: (a) Baking soda, a mild base, neutralises the milk's acid and raises the pH to slightly alkaline, keeping the milk fresh longer. (b) The added base must be neutralised by lactic acid first, so the milk takes longer to become acidic enough to set as curd.

Storage rule from the setting reaction. The reason for the special storage follows straight from the chemistry of how Plaster of Paris works. It is calcium sulphate hemihydrate, and it sets because it reacts with water to turn into gypsum.

The same reaction that makes Plaster of Paris useful is also its weakness during storage. If the powder is left exposed to a damp atmosphere, it takes up moisture from the air, and that water is enough to start the conversion to gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. The powder then slowly hardens in the container.

Once it has reacted with moisture and set, the plaster becomes a hard, solid lump that cannot be mixed into a paste any more, so it is no longer fit for making casts, moulds or smooth surfaces. To prevent this premature setting, Plaster of Paris is kept in a sealed, moisture-proof container that keeps air moisture away, preserving it as a dry, usable powder.

Answer: Because Plaster of Paris reacts with moisture from the air to form hard gypsum and set into a useless lump, it must be kept in a moisture-proof container.

Define it, then show it. A complete answer has two parts: a clear definition and two balanced examples. The definition is that a neutralisation reaction is the reaction between an acid and a base that produces a salt and water, in which the acidic and basic properties cancel out.

The simplest way to understand it is through the strong acid and strong base pair. When hydrochloric acid meets sodium hydroxide, the hydrogen ions of the acid join the hydroxide ions of the base to form water, while the leftover ions form the salt sodium chloride: NaOH + HCl → NaCl + H₂O. This is the classic example and is worth memorising.

A second example uses a different acid to show the same pattern. Sulphuric acid reacting with sodium hydroxide gives the salt sodium sulphate and water, needing two molecules of the base for one of the diprotic acid: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. Both examples produce a salt and water, confirming the general rule. Presenting the definition followed by two balanced equations is exactly what earns full marks.

Answer: Neutralisation is an acid reacting with a base to give salt and water; e.g. NaOH + HCl → NaCl + H₂O and 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.

Group the uses by the salt. The cleanest way to answer is to take each salt in turn and give two clear, separate uses, so the examiner can tick them off easily. Both salts come from common salt and are mildly basic, which already hints at uses like cleaning and neutralising acid.

Washing soda is sodium carbonate with ten waters of crystallisation. Its two standout uses are in industry, where it goes into making glass, soap and paper, and in water treatment, where it removes the permanent hardness of water by taking out the dissolved calcium and magnesium. It also serves as a household cleaning agent.

Baking soda is sodium hydrogencarbonate. One major use is in baking, where it is the key part of baking powder; on heating it releases carbon dioxide that makes cakes and bread rise and turn soft and spongy. Its other important use comes from its mild basic nature: it acts as an antacid to neutralise excess stomach acid, and it is used in soda-acid fire extinguishers. Listing two distinct uses for each salt gives a tidy, full-mark answer.

Answer: Washing soda: in glass, soap and paper industries, and for softening hard water. Baking soda: in baking powder for soft spongy cakes, and as an antacid (also in fire extinguishers).